1. Field of the Invention
The present invention relates to a data reproduction circuit for use in a data recording/reproduction apparatus. More specifically, the present invention relates to a data reproduction circuit for converting a read analog signal that was read from a magneto-optical disk or other medium by means of an optical head, into the original digital data.
2. Description of Prior Art
An example of data recording/reproduction apparatus, here a recording/reproduction apparatus for magneto-optical disk shown in FIG. 16, will be described hereinbelow.
A magneto-optical disk 1601 has a magnetic thin film as a recording medium on the surface of the disk which has a magnetic anisotropy such that the axis of easy magnetization is oriented vertical to the surface of the film. A laser beam 1602 is irradiated from an optical head 1603 onto the magnetic thin film. Due to this irradiation, the temperature of the irradiated spot is raised locally to decrease the coercive force Therefore, when a biased magnetic field is applied to the spot from a magnetic head 1604, the magnetization of the spot is inverted Here, the biased magnetic field may be applied only to the spot to be recorded, or may be preliminarily applied prior to the recording operation. If the direction and strength of the magnetic field applied from the magnetic head 1604, or the strength of the laser beam 1602 is controlled according to recording signals A sent from a modulation circuit 1605, digital data is magnetically recorded in a vertical direction as dots having a diameter of the laser beam 1602. When erasing recorded data, a magnetic field having a direction opposite to that adopted for the recording is applied.
Upon reproducing data recorded as described above, a laser beam 1602 weaker than that adopted for recording and erasing is irradiated onto the magnetic thin film of the magneto-optical disk 1601. The linearly polarized laser beam 1602 is reflected to have a polarization plane inclined according to the magnetized state of the magnetic thin film by the magneto-optical effect (Faraday effect or Kerr effect). Therefore, the inclination of the polarization plane of the reflected light is converted through an analyzer into an electrical signal by an optical detector housed in the optical head 1603 whereby a read analog signal B may be obtained
The read analog signal B is transmitted to a data reproduction circuit 1606 where it is converted into digital data C. The digital data C then passes through a PLL (Phase Locked Loop) 1607 and is demodulated in a demodulation circuit 1608. Modulation and demodulation are respectively performed in the modulation circuit 1605 and the demodulation circuit 1608 according to, for example, a modulation/demodulation method adopting the well-known 2,7 RLL code illustrated in Table 1. The 8/10 GCR code is also widely adopted.
TABLE 1 ______________________________________ (2,7 RLL code) Input bits Modulated bits ______________________________________ 10 0100 010 100100 0010 00100100 11 1000 011 001000 0011 00001000 000 000100 ______________________________________
A conventional data reproduction circuit will be described referring to FIGS. 17 to 26. FIGS. 17 to 19 illustrate a data reproduction circuit designed for an NRZI recording method while FIGS. 20 to 22 illustrate a data reproduction circuit designed for an RZ recording method.
A waveform of the read analog signal B (FIG. 17(d)) produced when the NRZI recording method is adopted is shown in FIG. 17. Modulated bits (FIG. 17(c)) are recorded onto a recording magnetic film 1701 (FIG. 17(a)) adopted as the magnetic thin film mentioned earlier, based on the recording signal A (FIG. 17(b)) and using the laser beam 1602 and the external magnetic field. The read analog signal B generated from the recording signal A is such that the leading and trailing edges thereof correspond to "1" of the modulated bits.
The conventional data reproduction circuit 1606 designed for the NZRI recording method will be discussed with reference to FIG. 18.
The read analog signal B is fed into an amplifier 1801 via a capacitor 1805. Interference in the waveform is then compensated for and S/N is improved in an equalizer and LPF (low-pass filter) 1802, and a reproduction signal D is sent to a non-inverted input terminal of a comparator 1803 and to an envelope detecting circuit 1804. The envelope detecting circuit 1804 emits a comparative voltage E corresponding to the center level of the envelope of the reproduction signal D to be sent to an inverted input terminal of the comparator 1803. The reproduction signal D and the comparative voltage E are compared in the comparator 1803 that releases digital data C.
FIG. 19 illustrates waveforms produced in the different sections shown in FIG. 18. With the NRZI recording/reproduction method, the leading and trailing edges of a recording mark 1901 (FIG. 19(b)) respectively coincide with a recording bit (FIG. 19(a)) "1". The read analog signal B (FIG. 19(c)) is produced by reading the recording marks 1901 by means of the optical head 1603.
The reproduction signal D is fed into the non-inverted input terminal of the comparator 1803. On the other hand, the comparative voltage E fed into the inverted input terminal of the comparator 1803 corresponds to the center level of the envelope of the reproduction signal D (FIG. 19(d)). Therefore, reproduction bits (FIG. 19(f)) coinciding with the recording bits may be obtained by having "1" correspond to the inverting positions of the digital data C (FIG. 19(e)) released from the comparator 1803.
The read analog signal B obtained when the RZ recording method is adopted, will be discussed now with reference to FIG. 20.
Modulated bits (FIG. 20(c)) are recorded upon the recording magnetic film 1701 (FIG. 20(a)) based on the recording signal A (FIG. 20(b)) by means of the laser beam 1602 and the external magnetic field. The difference with the recording NRZI method lies in the fact that, here, each peak of the read analog signal B (FIG. 20(d)) corresponds to the modulated bit "1".
A conventional data reproduction circuit designed for the RZ recording method will be discussed hereinbelow with reference to FIG. 21. The read analog signal B is fed into an amplifier 2101 via a capacitor 2105. Interference in the waveform is then compensated for and S/N is improved in an equalizer and LPF 2102. A first order differentiated signal F is sent via a differential circuit 2103 to a hysteresis comparator 2104 that releases digital data C.
FIG. 22 illustrates waveforms obtained with the configuration shown in FIG. 21.
With the RZ recording/reproduction method, each recording bit "1" (FIG. 22(a)) coincides with the center of a recording mark 2201 (FIG. 22(b)). The read analog signal B (FIG. 22(c)) may be obtained by reading the recording marks 2201 by means of the optical head 1603 The digital data C (FIG. 22(e)) is inverted as the first order differentiated signal F (FIG. 22(d)) fed into the hysteresis comparator 2104, goes beyond hysteresis levels Th.sub.1 and Th.sub.2. Therefore, reproduction bits (FIG. 22(f)) slightly lagging behind the recording bits may be obtained by having "1" correspond to the falling edges of the digital data C released from the hysteresis comparator 2104.
Lately, the development of magneto-optical recording/reproduction apparatuses has been actively pursued and high recording density together with high speed are demanded. Magneto-optical recording/reproduction apparatuses employing various modulation methods or recording/reproduction methods have been investigated and developed to meet this demand. However, the conventional data reproduction circuits discussed above suffer from the drawbacks that (1) high recording density, (2) compatibility, and (3) high speed are difficult to achieve. This will be covered hereinbelow.